During drilling operations it sometimes occurs that portions of the drill string become trapped or lodged within the well bore. These entrapped or lodged portions of the drill string are known to those skilled in the art as "fish". It at times becomes necessary to insert into the bore from the surface any one of a number of so-called "fishing tools" in order to retract the fish from the bore. The fishing tool is lowered into the bore at the end of a suitable work string for the purpose of securely engaging the fish to permit the extraction thereof.
It sometimes happens, however, that the fishing tool may stray from the axial centerline of the bore. Thus, the fishing tool may possibly miss engagement with the axial opening provided within the fish. To obviate such difficulties, the prior art provides a centralizer guide which is typically fixed to the work string by a shear pin. The purpose of the centralizer guide is to provide a flared element of sufficient diameter to engage the well bore and to maintain central axial alignment of the fishing tool as it is lowered into the bore.
Once the fishing tool has been guided to the proximity of the fish, the tool may enter the central opening thereof. Further axial advancement of the tool in the downstring direction severs the shear pin and permits the fishing tool to further penetrate the fish to secure it for extraction purposes.
However, if after the shear pin is broken the fish becomes disengaged, the centralizer is no longer effective as a guide. As a result, the entire string must be retracted to provide a new centralizer guide on the string.
Alternatively, it is possible that during the insertion of the string into the bore, the centralizer may encounter a protrusion or other imperfection within the surface of the bore sufficient to shear the pin prior to the engagement of the tool with the fish. Again, the entire work string must then be retracted and the centralizer guide replaced in order to successfully continue fishing operations. Of course, such retractions and reinsertions necessarily increase the cost of fishing operations.
The prior art, as exemplified by U.S. Pat. No. 3,785,690 (Hutchinson), assigned to the assignee of the present invention, discloses an overshot tool having a J-groove machined into the exterior surface thereof. The J-groove opens axially upwardly and receives a radially inwardly extending lug provided on the interior surface of an overshot. Various other prior art patents disclose substantially downwardly opening J-grooves. For example, U.S. Pat. Nos. 3,082,831 (LeBus, Senior), 3,223,169 (Roark) and 3,248,129 (Brown) disclose J-grooves opened at both axial ends thereof. U.S. Pat. Nos. 3,223,169 (Roark), 3,308,886 (Evans) and 3,386,764 (Evans) disclose downhole tools having J-grooves opening only in the downhole direction.
In view of the foregoing, it is advantageous to provide an overshot centralizer for guiding and maintaining a tool centrally and axially as the tool proceeds in the downstring direction within a well bore having a sleeve concentrically surrounding the tool and receiving therein a radially outwardly extending lug. It would be of further advantage to provide the sleeve with a substantially downwardly opening J-groove in which the lug is receivable. It would be of still further advantage to provide the downwardly opening J-groove such that the closed axial portion of the groove is spaced circumferentially from the downwardly opening lug thereof so that the tool may be alternatively disengageable from the centralizers to permit further axial advancement thereof or, engagable in a driving relationship with the centralizer to overcome obstructions with the bore.